Hydraulic power steerings usually employ hydraulic rotary slide valves, in which one valve element is connected with an input shaft of the valve and a second valve element is connected with an output shaft of the valve. The input shaft is connected with the steering wheel, and the output shaft is connected with a pinion which is part of a steering gear. The input shaft and the output shaft are usually coupled with each other by a torsion bar, so that they can be rotatably displaced relative to each other when the moment of resistance of the torsion bar is overcome. At the same time, the torsion bar specifies a central position of the valve, in which the valve provides no power assistance. When the input shaft and the output shaft and thus the two valve elements are rotatably displaced relative to each other, there is obtained a hydraulic flow in the one or other direction, which can be converted into a power assistance force.
At present, many efforts are made to develop the known power steerings to the effect that a power assistance force determined by a control unit can be superimposed on the power assistance force specified by the user. In this way, the steering characteristic of the power steering can be influenced in many ways, so that for instance the torque felt at the steering wheel can be influenced, the valve characteristic can be modified, the return to the central position can be influenced, a damping can be generated, an automatic actuation of the steering gear can be achieved, a mismatch of the central position can be compensated, a force resulting from the wheels, for instance due to a unilateral pressure drop in the tire or a braking operation with different friction values on the one and on the other side of the vehicle, can be counteracted, or non-uniform wheel loads can be attenuated. The steering characteristic of a rotary slide valve can be influenced in that one of the valve elements, usually the valve sleeve connected with the output shaft and hence with the pinion, is not firmly mounted at the output shaft, but can be rotatably displaced relative to the same. The displacement in rotation of the valve sleeve relative to the output shaft can be caused by an electric motor or by coils which adjust the valve sleeve by electromagnetic forces.
However, such active rotation of the valve sleeve raises two problems. The first problem consists in that the position of the valve sleeve must be controlled with a very high angular accuracy. Since the output shaft can usually rotate by four revolutions between the two steering stops, the drive means for the valve sleeve must be able to displace the same in rotation over an angular range of 3 to 4 steering wheel revolutions with an accuracy in the order of 0.02°. This requires a very expensive angle sensor, which can operate with the required accuracy. The second problem relates to the failure safety. In the case of a failure of the drive means, the angular position of the valve sleeve can no longer be predicted, so that the power steering can no longer operate properly. In the worst case, the driver looses control over the vehicle.